In the proportioning of a miscible solute in a water solvent under continuous flow, objectionable ratio variation of over 50% can occur in the simple low cost conventional systems because of pressure variations or temperatures in either the solvent and/or solute supply. The conventional practice particularly with portable units has predominantly become the use of either one of two designs in single stages as related to the relative pressure of the solute at the mixing chamber, both having their advantages and disadvantages:
(1) High Vacuum, restricted solute flow: A mixing system which develops a deep vacuum condition approaching zero p.s.i. absolute in the first mixing stage, for the purpose of conducting the solute through a flow restriction at the mixing chamber level in order to meter thereby the amount of solute supplied for a desired ratio proportioning that is related to the flow of solvent into the mixing chamber. Advantage: The use of high vacuum reduces the significance of lift height variations on the solute supply pressure to the flow restriction; or, Disadvantage: proportioning accuracy is affected by solvent pressures and the system is not suitable for low ratios due to difficulty of maintenance of high vacuum in such cares.
(2) Low Vacuum, free solute flow: A mixing system in which the pressure acting upon the solute in the confluence chamber is essentially the free flow low vacuum lift height variations of the solute. Advantage: Proportioning is essentially independent of variations in solvent pressure. Disadvantage: lift height variations adversely affect proportioning accuracies for single stage mixing systems and high ratios are difficult to achieve in a single stage due to close mechanical tolerances required for proportion orifices.
In applications where hot solvent and/or solute is being proportioned and used, vaporization of either liquid induced by low pressure can change the proportioning ratio.
Since the degree of vaporization is related to the extent of vacuum existing in the primary mixing gap, this vaporization effect is much more deleterious in the high vacuum mixing system, and thus the minimal effect of vaporization is another advantage of a low vacuum system.
Although either system is quite accurate if the exact design solute and solvent pressures are used and their relationship remains constant, only a small percentage of the commercial market would be satisfied and the danger of improper solution when higher temperature liquids are used due to the vaporization effect still confronts most of the users.
Thus, at conventionally tolerated performance ratios which are available, the solute metered flow system is generally used as limited to ratios above a 1 to 10 and the free flow solute system limited to ratios below 1 to 24. Two-stage conventional high vacuum gap proportioners may provide a better pressure output efficiency, but they perform no better than single stage proportioners with respect to ratio variations caused by the solvent pressure and solvent and/or solute temperatures.